Analysis of Chemical Modification Mechanism and Rheological Properties of Polyphosphoric Acid Modified Asphalt

In order to study the chemical modification mechanism and rheological properties of polyphosphoric acid (PPA) -modified asphalt, asphalt modified with different PPA contents were characterized by four-component test, atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). In the test, changes in asphalt chemical composition and colloidal structure were analyzed for different PPA contents, and infrared spectra were fitted with a Gaussian function. The reaction mechanism of PPA and matrix asphalt was also discussed. Based on dynamic shear rheometer (DSR) test and bending beam rheometer (BBR) test, rheological index G*/sinδ and S/m were used to evaluate the modification effect of PPA on asphalt. The results show that, with an increase in PPA content, both large and small honeycomb structures increased in the three-dimensional topography seen in the atomic force microscope (AFM). In a certain space range, some of the micelles in the asphalt are connected each other to form interlocking skeleton structures, and locally form dense spatial network structures. The added PPA does not chemically react with the functional groups in the functional-group area of the infrared spectra (3 100–2 750 cm−1, 1 800–1 330 cm−1), and the structure is very stable. However, there is an obvious new absorption peak below 1 330 cm−1 in the fingerprint area, that is, the chemical reaction between PPA and the matrix asphalt generates a new compound, inorganic phosphate. Infrared spectra of PPA-modified asphalt with different contents were fitted by a Gaussian function, which makes up for the limitation that the absorption intensity information of each superimposed functional group cannot be obtained directly from the original infrared spectra. Results of this qualitative analysis were further verified by quantitative analysis. The addition of PPA can effectively improve the high and low-temperature performance of asphalt, and the lower the temperature is in the negative temperature zone, the more obvious the improvement is. When PPA content is more than 1%, the improvement of asphalt low-temperature performance is not obvious.